Can dietary C:N ratio influence water quality and microbiology in recirculating aquaculture systems?

Abstract

The potential impacts of diet composition on water quality in recirculating aquaculture systems (RAS) have been studied only to a limited extent. This study examined the effects of different dietary C:N ratios on physiochemical water quality and associated microbiology in 12 pilot scale freshwater RAS with rainbow trout (Oncorhynchus mykiss). By adjusting the proportions of protein, carbohydrate, and fat, four diets with different C:N ratios—fully embracing the span observed in commercial diets—were formulated (CN8.5, CN9.3, CN10.0, and CN12.2). Each diet was administered to three separate RAS for 4 weeks at a daily feed loading of 1.67 kg/m3 make-up water. At the end of the trial (in week 4), concentrations of nitrite-N, nitrate–N, and dissolved COD, and microbial activity were significantly lower and water clarity was simultaneously higher in systems fed the CN12.2 diet as compared to the other diets. Microbial community compositions in system water and biofilter biofilms and the overall microbial abundance in system water were not affected. This was likely due to the molar C:N ratios in system water not correlating to the dietary C:N ratios and being very low (0.27–0.34) throughout all dietary C:N levels, suggesting that microorganisms were carbon limited in all RAS. Altogether, the results demonstrate that changing dietary C:N ratios can affect physicochemical water quality and microbial activity in RAS. In contrast, changing dietary C:N ratios did not affect water C:N ratios or water or biofilter microbial community composition at least within the span of dietary C:N ratios observed for commercial juvenile rainbow trout feeds.